1. Field of Art
The present invention relates to a process for the preparation of a highly homogeneous fluoride glass which may be used as a material for optical fibers, laser glasses, glass coatings and lens, and also to a process for the preparation of a fluoride optical fiber and a preform therefor which can provide a long optical fiber having low transmission loss.
2. Prior Art Statement
Fluoride glasses have heretofore been known as optimal materials for optical fibers, glass coatings or films, laser glasses and lens because of their excellent transmission properties within the infrared region range, and are glass materials for optical fibers which are better than silica glasses because they have transmission losses of less than 10.sup.-2 dB/km which is superior to the silica glasses.
U.S. Pat. No. 4,718,929 discloses a CVD (chemical vapor deposition) process for preparing metal halides. This prior publication discloses a CVD process for preparing a metal halide glass material which may be used to produce optical fibers used in the infrared region or other optical members, wherein a .beta.-diketone complex containing a fluoride of Be or Al is decomposed in a gaseous phase without using highly corrosive hydrogen fluoride (HF) gas to deposit a BeF.sub.2 (85 to 100 mol%)/AlF.sub.3 (15 to 0 mol%) glass on a substrate. However, strong toxicity and deliquescence of BeF.sub.2 system glasses are an obstacle to practical application thereof. Moreover, the specification of this prior patent fails to describe the preparation of fluoride glasses containing Ba.
U.S. Pat. No. 4,378,987 discloses a low temperature process for the preparation of an optical fiber in which an organic metal compounds is used. In this prior art process, a gaseous halogenation agent, such as BF.sub.3, SiF.sub.4, COF.sub.2, HF, HCl, SiCl.sub.4 or BCl.sub.3, is used for preparing a metal halide so that the halogenation agent is reacted with a gaseous reactant of an organic metal compound to produce a glass material made of a solid metal halide. However, the specification of this patent does not disclose the use of complexes of Ba and .beta.-diketones.
In conventional processes, fluoride glasses are generally produced through a so-called baich melting process in which solid materials are used. In the batch melting process, solid materials are first weighed, followed by pulverization and mixing, and then the mixed materials are melted in a batch. Thereafter, the melt is rapidly cooled to produce a glass.
However, the process described in the preceding paragraph has the problems that the materials are apt to be contaminated with transition metals, such as iron, nickel, copper, chromium, cobalt, during the weighing and pulverization steps, and that the materials tend to absorb moisture. Since the impurities including transition elements have absorption peaks within the infrared region, they cause absorption loss within the infrared region of the resultant product. Absorbed water or moisture causes scattering loss. There is also a problem that the wall of a used melting apparatus is corroded during the step of melting the glass, leading to contamination of impurities. A further disadvantage is that a large size fluoride glass product cannot be produced since the melt is cast into a mold followed by rapid cooling.
Other processes disclosed for the preparation of a preform for optical fibers include a built-in-casting process (reference should be made to Japanese Journal of Applied Physics, Vol 21, No. 1, pp 55 to 56 (1982)), and a modified built-in-casting process.
Moreover, as has been described above, since a melt is cast into a mold, a large size preform cannot be produced. Furthermore, the known casting processes for production of a core cladding structure by a casting process include a process wherein a cladding glass melt is flown out before the cladding glass melt has not solidified and then a core glass melt is cast (such a process being referred to as build-in-casting process), and a process wherein a core glass melt is cast above the cladding glass melt and the cladding glass melt is flowed out from the lower end as the core glass melt is in the semi-solidified state so that the core glass is introduced into the center portion of the cladding glass (modified built-in-casting process). However, these known processes have the disadvantages that a preform for fibers which has unform core/clad diameter ratio cannot be produced and that the refraction index profile of the resultant preform for fibers cannot be controlled.
On the other hand, the CVD process has been known as a process for preparing silicaglass optical fibers. It is suited for the synthesis of high purity homogeneous glass. However, when a glass is prepared by the CVD process, compounds of elements constituting the product glass must be heated to vaporize. Since the fluoride glass is mainly composed of compounds of alkali metals, alkaline earth metals and rare earth elements which are scarcely have sufficiently high vapor pressures at a relatively low temperature, it was difficult to prepare fluoride glasses by the CVD process.